Problem with 4 DVD Writer Tower

I have 4 asus 4x dvd-r writers in a tower with the following specs :
winxp-pro/amd 2ghz+/512 ram/120gb samsung and i use the latest version of nero to write dvd-video simultaneously to all 4 dvd writers.
i am having the foll problem:
for the first time nero succesfully writes to all 4 dvd-r discs perfectly but as soon as i put another lot to burn nero first starts to write the lead in and fails 3 out of 4 writers giving error " power calibration error" (it continues to write and burns succesfully 1 out of 4 writers)
all the hardware is brand new and has no other problems.

pls help